Two peptides of eight (T2) and 10 (T1) residues corresponding to the branched moiety of ubiquitinated histone H2A have been synthesized and used for raising specific antibodies in rabbits. Antisera to peptide T1 reacted in ELISA with T1 and with H2A but not with ubiquitin; antisera to peptide T2 reacted with T2 but not with H2A or ubiquitin. When tested in immunoblotting, both peptide antisera reacted with ubiquitinated H2A but not with unconjugated H2A or with ubiquitin. Sera from patients with systemic lupus erythematosus (SLE) were shown previously to react with ubiquitin in ELISA and immunoblotting. When tested for their ability to react in ELISA with synthetic peptides T1 and T2, 96% of the SLE sera (diluted 1:500) that recognized ubiquitin also reacted with peptide T2. Of the SLE sera that did not react with ubiquitin, only 13% possessed antibodies able to bind peptide T2. Antibodies from seven SLE sera, purified on a T2-immunoadsorbent column, were also able to react either with H2A, and in three cases also with ubiquitin.